Echo cancelling systems are well known in the prior art, and many diverse systems have heretofore been utilized and/or suggested.
In conjunction with such systems, it has been heretofore suggested that an adaptive algorithm can be utilized to enable a transversal filter to act as a self-controlling waveform conditioner, and resulting systems have been heretofore used for both data transmission and voice echo control. For signals in a dispersive transmission media (i.e., channels with linear distortion such as a non-flat gain or non-linear phase), use of such systems has been found to permit a higher level of utilization (i.e., permits a faster data rate or less echo) than would otherwise be feasible.
Heretofore, it has been common to achieve echo cancellation through utilization of stepwise linear approximations to logarithmic code spaces for both coding the arriving speech signal and for storing the termination parameters. A primary consideration for use of such an approach is that digital adding circuits (which are much less complex than digital multiplication circuits) can be conveniently used in the digital multiplication process since the logarithm of the product of two numbers is the arithmetic sum of their separate logarithms. Companding code spaces are somewhat more complex to generate, however, than are linear, or uniform, code spaces and, in addition, require added circuitry to linearize the code space before the pairwise product terms can be added together to build up the necessary echo sample value. FIGS. 11a and 11b typically illustrate differences between linear and companding, or logarithmic, code spaces.
A self-adaptive echo canceller is shown in U.S. Pat. No. 3,500,000, and, in conjunction therewith, the adaptive algorithm, as used in this invention, is set forth. While the system, as shown in U.S. Pat. No. 3,500,000, is the basic approach utilized in this invention, there is no teaching of use of linear code spacing or circuitry which enables effective use of such spacing for echo cancellation.
An improved adaptive echo canceller is shown in U.S. Pat. No. 3,836,734, but, again, linear code spacing and/or associated circuitry for effective use is not utilized and error correction is achieved by multi-element threshold detectors in the adaptive control loop for varying each component of the termination signal by different amounts.